This invention relates to a pressure swing adsorption (PSA) process for purifying impure gas streams containing more than 50 mole % hydrogen, and more particularly to such a process for the production of high purity hydrogen from various hydrogen-containing feed mixtures such as synthesis gas. The improved process provides higher hydrogen recovery and lower adsorbent inventory as compared with previously known PSA processes for hydrogen production.
The need for high purity ( greater than 99.9%) hydrogen is growing in the chemical process industries, e.g. in steel annealing, silicon manufacturing, hydrogenation of fats and oils, glass making, hydrocracking, methanol production, the production of oxo alcohols and isomerization processes. This growing demand requires the development of highly efficient separation processes for H2 production from various feed mixtures. In order to obtain highly efficient PSA separation processes, both the capital and operating costs of the PSA system must be reduced.
One way of reducing PSA system cost is to decrease the adsorbent inventory and number of beds in the PSA process. In addition, further improvements may be possible using advanced cycles and adsorbents in the PSA process. However, H2 feed gas contains several contaminants, e.g. CO2 (20% to 25%) and minor amounts of H2O ( less than 0.5%), CH4 ( less than 3%), CO ( less than 1%) and N2 ( less than 1%). Such a variety of adsorbates at widely varying compositions, combined with the high purity ( greater than 99.9%) requirement for H2, presents a significant challenge to efficient selection, configuration and amount of adsorbents in each layer of the bed to achieve an efficient H2-PSA process.
There are a variety of known processes for producing hydrogen. For example, FIG. 1 of the accompanying drawings shows the steam reforming of natural gas or naphtha wherein a feedstock, e.g. a natural gas stream 11, is compressed and fed to a purification unit 12 to remove sulfur compounds. The desulfurized feed is then mixed with superheated steam and fed to a reformer 13 to produce primarily H2 and CO. The effluent stream from the reformer is sent to a heat recovery unit 14, then to a shift converter 15 to obtain additional H2. The effluent from the shift converter is cooled and recovered in unit 16. The effluent, synthesis gas stream 17, having on a dry basis a composition of about 74.03% H2, 22.54% CO2, 0.36% CO, 2.16% CH4, and 0.91% N2 is then routed to a PSA purification system 18 to produce a high purity hydrogen product stream 19.
Representative prior art PSA processes for hydrogen purification include the following: (1) Wagner, U.S. Pat. No. 3,430,418, (2) Batta, U.S. Pat. No. 3,564,816, (3) Sircar et al., U.S. Pat. No. 4,077,779, (4) Fuderer et al., U.S. Pat. No. 4,553,981, (5) Fong et al, U.S. Pat. No. 5,152,975 and (6) Kapoor et al., U.S. Pat. No. 5,538,706.
The adsorbers in hydrogen PSA processes have been conceptually divided into multiple zones, depending upon the particular contaminants to be removed in the successive zones. For example, in Wagner (U.S. Pat. No. 3,430,418) a combination of two types of adsorbents is used, i.e. activated carbon for the removal of H2O and CO2, and calcium zeolite A for removal of CO and CH4 (see Example 1). The Wagner patent describes an eight-step PSA cycle for hydrogen purification. At least four beds are used in the process; following the bed-to-bed equalization step, each bed undergoes a co-current depressurization step prior to countercurrent blowdown to recover void space gas for purging of another bed.
Batta (U.S. Pat. No. 3,564,816) describes a twelve-step PSA cycle using at least four adsorbent beds and two pressure equalization stages for separating hydrogen-containing gas mixtures contaminated with H2O, CO2, CH4 and CO produced in the steam reforming of natural gas. In the Batta process, a co-current depressurization step follows the first bed-to-bed equalization step to recover void space gas for purging of another bed. A second bed-to-bed equalization step is used prior to the countercurrent blowdown step in the PSA cycle.
Sircar, (U.S. Pat. No. 4,171,206), discloses a PSA process in which a crude hydrogen stream (such as the gaseous effluent from a shift converter of a hydrocarbon reforming plant) flows through a first bed of activated carbon (effective for the removal of CO2), and then through a second bed of 5A zeolite (effective for the removal of dilute impurities such as CH4 and/or CO) to produce high purity ( greater than 99.9%) hydrogen.
Golden et al, (U.S. Pat. No. 4,957,514), discloses the purification of hydrogen using a barium-exchanged Type X zeolite to remove CO, CH4 and N2 contaminants. According to Golden, the preferred BaX zeolite is one in which 60 to 100% of the sodium cations of a NaX zeolite have been replaced by barium cations. Golden compares the adsorbent requirements using BaX (96% Ba, 4% Na), CaX (98% Ca, 2% Na), Ca/SrX (50% Ca, 50% Sr), and commercial 5A zeolites in hydrogen purification processes. For a given feed flow rate and H2 purity, the quantity of zeolite required in the hydrogen purification process is lowest when BaX is used. Also, Golden ranks the adsorbents in the order BaX greater than Ba/SrX greater than 5A greater than SrX greater than Ca/SrX greater than CaX for CO or CH4 adsorption. In particular, CaX is ranked the lowest with respect to the removal of CO and CH4 impurities.
Scharpf et al, (U.S. Pat. No. 5,294,247), discloses a vacuum PSA process for recovering hydrogen from dilute refinery off gases, preferably containing less than 60% hydrogen. The patent discloses the use of six adsorbent beds, each of which contains a layer of activated carbon, a layer of 13X zeolite, a layer of 5A zeolite and a layer of CaA zeolite or calcium exchanged X zeolite. This four-layer arrangement is described as useful for the removal of large feed concentrations ( greater than 1%) of CO and CO2.
More recently, Bomard et al in International Patent Application W097/45363, disclosed a method for separating hydrogen from a gas mixture that contains CO and other impurities such as CO2 and hydrocarbons. In the Bomard application, the feed mixture is passed into a first selective adsorbent (e.g., activated carbon) to remove CO2 and hydrocarbons. It is then contacted by a second adsorbent, a faujasite-type zeolite with at least 80% lithium exchange, to remove primarily CO impurity and to produce high purity hydrogen. In addition, a third adsorbent (5A zeolite) may be placed between the first and second adsorbents to remove nitrogen if N2 is also present in the feed mixture.
It is also known to utilize PSA processes for the selective adsorption of N2 from air to obtain O2-enriched gases. Berlin, (U.S. Pat. No. 3,313,091), describes the use of strontium-substituted type X zeolites in such a process and stipulates that Ca2+, Sr2+, and Ag+ are the preferred exchanged cations with Sr2+ being most desirable. In the case of Type A zeolite, Ca2+, Mg2+ and Ag+ are preferred.
Coe et al, (U.S. Pat. No. 4,481,018), also discloses PSA air separation processes involving N2 and O2 separations using calcium-exchanged forms of zeolite X, and showed enhanced PSA process performance with increasing calcium content. However, Chao (U.S. Pat. Nos. 5,698,013 and 5,454,857) discloses for calcium-exchanged forms of zeolite X a peak performance in air separation below maximum calcium content. In particular, the peak performance occurs when the degree of calcium exchange is in the range of 60 to 89 equivalent percent, and the SiO2/AlO2O3 molar ratio is in the range of 2.0 to 2.4.
The operating conditions used in H2 PSA processes are distinct from those used in PSA processes for O2 production from air. The adsorption pressure is typically less than 2.5 bars in VPSA air separation, while the adsorption pressure is 5 to 20 bars in H2 PSA.
The amount of N2 in the feed streams of these two processes is significantly different, e.g. the N2 fraction in air is about 78 mole %; whereas, in hydrogen purification, the proportion of N2 in the feed stream is typically less than 1-3 mole %. Consequently, the N2-selective adsorbents disclosed in the aforementioned references for air separation must perform in different N2 partial pressure regions of the isotherm than those required for the function of purification in H2 PSA processes. Furthermore, the N2 differential loadings in O2 VPSA and H2 PSA processes are quite different for a given N2-selective adsorbent (e.g., zeolite). Additional complications in the design of H2PSA processes result from the competitive adsorption and diffusion rates of each adsorbate in the mixture. In selecting improved adsorbents and designing H2 PSA processes, due consideration must be given to all of the foregoing issues.
The adsorbent typically used heretofore in H2 PSA processes is 5A zeolite, obtained by base exchange of Ca (about 75%) with the sodium ion present in 4A zeolite.
It is among the objects of the present invention to provide an improved PSA process for the production of hydrogen from an impure gas stream containing more than 50 mole % hydrogen, such improvements realized as increased hydrogen recovery, reduced adsorbent and lower capital and operating costs. Other objects and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing.
In accordance with the present invention, an improved PSA process is provided for purifying a gas stream containing more than about 50 mole % hydrogen, which comprises adsorbing substantially all of the nitrogen in such stream by passing it at superatmospheric pressure through an adsorbent bed containing CaX, LiA or LiX-type zeolite adsorbent having SiO2/Al2O3 mole ratio within the range of 2.0-2.5, and recovering purified hydrogen as product from the adsorbent bed. By providing additional adsorbent layers to remove other impurities such as H2O, CO2,CH4 and CO upstream of the CaX, LiA or LiX zeolite adsorbent, a high purity (xe2x89xa799.9%) hydrogen product is recovered.
In accordance with a further feature of the invention, substantially all of the CO2 is first removed, i.e. leaving less than about 0.15 mole %, preferably less than about 0.10 mole % CO2 for subsequent passage into the zeolite layer where this residual CO2 is removed along with the N2. Configuring the adsorbent bed according to this contaminant removal strategy increases H2 recovery and reduces the quantity of zeolite required for N2 removal compared to prior art PSA processes for H2 production.
The preferred adsorbents utilized in the process of the present invention are CaX zeolites, most desirably CaX (2.0), a zeolite of the faujasite type exchanged at least 90% with calcium and having a SiO2/Al2O3 molar ratio of 2.0. CaX (2.0) allows the processing of more feed gas per unit weight of adsorbent at a given P/F (purge to feed) ratio than other N2-selective adsorbents. Accordingly, by using CaX (2.0) the amount of the adsorbent required for removing N2 may be significantly reduced, i.e. reducing the bed size factor (BSF). Moreover, this reduction in bed size factor results in lower H2 losses during regeneration of the adsorbent bed. This in turn leads to higher hydrogen recovery than may be obtained with other N2-selective adsorbents.